JPS59190332A - Production of galvanized steel plate for ultradeep drawing having extremely good secondary processability - Google Patents

Abstract

PURPOSE:To produce a galvanized steel plate for ultradeep drawing having extremely good secondary processability by galvanizing a steel plate contg. C, Si, Mn, P, Al, N, B, Ti, Nb, etc. in specific compsn. and compsn. relation. CONSTITUTION:A steel which consists of <=0.005% C, <=0.8% Si, <=1.0% Mn, <=0.1% P, 0.01-0.1% Al, <=0.005% N, and other unavoidable impurities, is further added with B, Ti and Nb in combination and contains the components satisfying the conditions 48/14(N%-0.003%)<=Ti%<=48/12C%+48/14N%, Nb%> 2C%, 0.003% <=Nb%<=0.04, Ti%+Nb%<0.06% is subjected to hot rolling at <=1,300 deg.C heating temp. for hot rolling then to descaling and cold rolling followed by continuous annealing at the temp. of the crystallization temp. or above and the Ac3 point or below and is further subjected to galvanizing, by which the galvanized steel plate having excellent deep drawability and the performance to suppress the generation of cracking by secondary processing in spite of severe deep drawing is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-dip galvanized steel sheet for ultra-deep drawing which has extremely excellent secondary workability.

Conventionally, ultra-low carbon steels with added carbonitride-forming elements have been developed as deep-drawn steel sheets for continuous annealing, but such steel sheets become brittle when subjected to secondary processing after severe deep drawing. It has a tendency to destroy, especially P, 81. When manufacturing high-strength steel sheets by adding Mn, etc., P and Si have a strong property of embrittling the steel sheet, so the secondary work embrittlement described above is extremely likely to occur. B has crystal grains at the same level as C. However, as a result of actual research and study, the present inventors obtained the following new knowledge that the effect of B addition on steel sheet material varies depending on the steel type and manufacturing conditions. The present invention was completed based on the above.

When B is added to Ti-added ultra-low carbon steel, the secondary workability tends to be improved, but the improvement effect is relatively small, and the deep drawability ( r value) and ductility (Et) are extremely large. In Ti-added steel, Ti is O and N in the pot.

Since it has a very strong tendency to form precipitates with S and C, the grain boundaries are extremely clean and the grain boundary strength is very weak. Even when B is added, the brittle grain boundary properties remain, so the effect of improving secondary workability is relatively small.

When B is added to Nb-added ultra-low carbon steel, the effect of improving secondary workability is small if the amount of B added is small;
On the other hand, when the total amount of B added increases to the extent that the effect of completely improving secondary workability appears, the r value and
The deterioration of Et is extremely large.

The reason for this phenomenon is that in the case of Nb addition, B has a greater tendency to form nitrides than Nb and AL, so the added B forms BN and improves secondary workability. Since only a small amount of solid solution B exists in the form of effective B, the effect is small when a small amount of B is added.

In order to ensure all the B present as solid solution B, it is necessary to add an amount of B that is greater than the amount that forms BN and N.

Since BN has a strong tendency to deteriorate the r value and Et-tz, the material quality deteriorates significantly and is not preferred as a steel plate for deep drawing. Furthermore, since the amount of B that becomes BN is determined by the amount of N in the steel, the amount of B added needs to be increased in consideration of safety, taking into account all the fluctuations in the N content during actual manufacturing. Even when B exists as a solid solution, B tends to deteriorate the material, so it is necessary to increase the amount of B added.
B-added steel has major drawbacks such as material deterioration and material fluctuation.

The present inventors have obtained new knowledge that, in addition to the above-mentioned problems caused by the addition of B, conventional ultra-low carbon deep-drawn steel sheets for p-use have the following drawbacks.

In steel with the sole addition of T1, when the amount of Ti added is less than the equivalent amount of C and N, carbides (Tic) are finely precipitated, resulting in significant deterioration in material properties such as ductility, yield strength, deep drawability, and aging resistance. It tends to deteriorate. Therefore, is the material affected by deep drawing properties? In order to obtain Ti, it is necessary to make the amount of Ti added equal to or more than the equivalent amount of C and N. In this case, there is almost no solid solution C in the steel sheet, resulting in extremely poor secondary workability, and furthermore, if the amount of Ti added is Because of this, the phosphate treatment performed as a base treatment for painting deteriorates significantly.

For Nb-added steel, there are restrictions on hot rolling coiling temperature, annealing temperature, and cooling rate after annealing. For Nb-added steel, high-temperature coiling (coiling temperature ≧700°C) during hot rolling is mandatory. At normal coiling temperatures, the complete recrystallization temperature becomes very high, and in the continuous annealing furnace's possible temperature range (usually about 850°C or lower), some unrecrystallized parts remain. There are large fluctuations. This is related to the formation of A/J and NbC, and is thought to be because these precipitates do not have a sufficient size in the hot rolled sheet, thereby suppressing recrystallization. When high-temperature winding is performed, the temperature of the hot-rolled coil is about 800 to 85, excluding the longitudinal ends of the coil.
Various reports have shown that a steel plate with a high r value can be obtained at an annealing temperature of 0°C. This is AtN, NbC
This is related to the formation of large-sized precipitates in the hot-rolled sheet during high-temperature coiling. However, high-temperature winding not only drastically reduces pickling efficiency due to thick scales, but also because the cooling rate at the ends of the coil is fast, the material remains at the same temperature as the normal winding temperature. Since this is not possible, the reduction in yield is particularly large in Nb-added steel. The second is annealing V after cooling
・It is a matter of temperature and cooling rate after annealing. For example, JP-A-55-141526 and JP-A-55-141555.
As stated in the publication, when annealing at a high temperature (approximately 900° C. or higher), solid solution C and N are formed because AtN and NbC are redissolved, and slow aging cannot be achieved unless slow cooling is performed after annealing. Therefore, it becomes a problem from the viewpoint of operability and economy.

The present invention relates to the production of a hot-dip galvanized steel sheet for ultra-deep drawing, which eliminates the drawbacks of these conventional Ti-added steels and Nb-added steels and has excellent secondary workability.

In other words, the present invention has excellent deep drawing properties and the ability to withstand severe deep drawing processes without causing secondary processing cracks even in the unlikely event of occurrence.
This was done for the purpose of providing a method for manufacturing hot-dip galvanized steel sheets that is insensitive to hot-rolling and winding conditions, and the rationale behind this is as follows.

(g c: 0.005% or less, Si: 0.8% or less, Mn: 1.0% or less, p: oyt% yield, ht: o
, 01 to 0.1%, N: 0.005% or less and other impurities, and B, Ti, Nb
When F complex is added, B is 30 ppm.
Add within the following range, 48 4
8 Tt is TT [: N (%) - 0,00a %:)≦
It is contained within the range that satisfies the condition of Tt (1≦TTc) + T7 N (%), and Nb is contained within the range that satisfies the condition of Nb (%) > 2 c (z
J) and has a content within the range that satisfies 0.003%≦Nb (0.04%) and a composition that satisfies Tl + Nb (%) < o, o 6. After rolling, followed by descaling treatment and cold rolling, A cs
1. A method for producing a hot-dip galvanized steel sheet for ultra-deep drawing, which has extremely excellent secondary workability, characterized by continuous annealing at a temperature below 100 mL, and then hot-dip galvanizing.

(2) After hot rolling, the coiling temperature is 600°C or higher and 800°C or lower, followed by cold rolling and annealing at a temperature higher than the recrystallization temperature and lower than the Ac5 transformation point in a continuous hot-dip galvanizing line. 700℃ to 500℃ after treatment
The method according to claim 1, wherein cooling is performed at a cooling rate of 150° C./sec or higher over the entire temperature range up to 150° C./sec, and then hot-dip galvanizing is performed.

The basic principles of the invention are fully described below. The fundamental idea of the present invention is to significantly increase the grain boundary strength by concentrating solid solution B and solid solution C present in the steel sheet at the grain boundaries, thereby providing extremely excellent secondary workability. Furthermore, in order to exhibit the above effects by converting the B added into the steel into a solid solution B, Ti'(
Add i7 complex. The effect of composite addition of Ti is that of N in steel.
'! r By precipitating and fixing as TiN, the added B completely prevents the formation of BN'i and forms solid solution B. Therefore, the amount of B added is effective even in small amounts, and the deterioration of ductility (Et) and deep drawability (r value) due to B addition can be suppressed. Furthermore, the effect of the combined addition of Nb is to precipitate and fix a portion of the C in the steel as INbc, thereby reducing the amount of solid solute C so as to substantially prevent aging.

The reason why the steel sheet according to the present invention has superior deep drawing properties and secondary workability compared to conventional steels with only T1 added and steels with only Nb added is because of the combined addition of a small amount of B, Ti, and Nb'r. This is due to a number of reasons. In other words, N in the steel is already precipitated and fixed as TiN in the hot rolling furnace due to the small amount of Ti added in a composite manner. Since TiN is extremely stable as a nitride, it does not change at all during the hot rolling, cold rolling, and recrystallization annealing processes. Therefore, its precipitation form varies depending on the hot rolling coiling temperature, continuous annealing temperature, and subsequent cooling rate. remains unchanged.

Since B added to steel has a smaller tendency to form nitrides than Ti, it exists as solid solution B, and has the effect of increasing grain boundary strength with a small amount of addition. On the other hand, when B is added to Nb-only steel, the tendency of nitride formation is
, Since B is larger than At, the added B is
Form N. Therefore, when the amount of B added is small, there is no solid solution B that has the effect of completely improving secondary processability (B is
(Existence as N), there is no effect on improving secondary workability. In order to secure all the B present as solid solution B, it is necessary to add more than the amount of Bi that forms N and BN.
Since B has a strong tendency to deteriorate the r value and Eti, increasing the amount of B added causes material deterioration and is not preferable as a steel sheet for deep drawing. Furthermore, when Bt is added to a steel with only Ti added, as is known, the r value and El deteriorate significantly compared to a material without B addition. Furthermore, Ti is O, N, S, C in steel.
Since the tendency to form precipitates is extremely strong, the grain boundaries are extremely clean and the grain boundary strength is very weak. Therefore, in order to improve the properties of brittle grain boundaries by adding B, it is necessary to increase the amount of B added, but this is not preferable from the viewpoint of material quality. On the other hand, Ti added according to the present invention is N i TiN
It plays the role of precipitating and fixing as sb, and does not cause the drawbacks seen in the above-mentioned Ti-only steel.

The results of repeated comparative studies of the materials of the steel according to the present invention in which the amount of B added was varied in various amounts and the above-mentioned steel with only Tj and Nb added showed that the steel sheet according to the present invention had the best ductility and deep drawability. It also showed clear superiority in terms of secondary processability.

@ according to the present invention is one in which Nb is added in combination with B + Tj, but due to the coexistence of Ti and Nb, p (Ti
, Nb)C, etc. are formed and begin to precipitate before finishing during hot rolling (i.e., in the austenite temperature range), so a material with good quality can be obtained even at a low coiling temperature.

The reason why the steel sheet obtained according to the present invention has superior material properties compared to the conventional steel with only Nb added is that it is possible to stabilize the secondary processing by adding a small amount of B to the precipitation and fixation of N as TiN. ■ The fact that N is already precipitated in the hot rolling heating furnace as TjN, eliminating the factor of variation in the coiling temperature caused by N. ■ The combined addition of Ti and Nb (Tl, Nb) The advantage of this method is that composite precipitates such as Nb) and C can be formed before finishing, and a material with good flexibility can be obtained even at a low winding temperature.

Next, the composition range of the steel according to the present invention will be described.

First, it is necessary to add B in an amount of tit 30 ppm or less. The addition of B to the steel according to the present invention has the effect of improving secondary workability, and this effect is due to B existing in a solid solution state. In the steel according to the present invention, pN is precipitated as TiN by the combined addition of Ti, so that the added B becomes solid solution B, and even a small amount is sufficiently effective. As the amount of B added increases, the r value and Et tend to deteriorate slightly.From the characteristics of the steel according to the present invention, which is a steel plate for ultra-deep drawing, the upper limit is set at 30 ppm. The most desirable addition amount is 2 ppm or more and 25 ppm or less.

Ti is added to fix N and eliminate its harmful effects, and it is necessary to add more than HCN (@-o, o O3es).In other words, the lower limit of the amount of Ti added is calculated as Tl. The amount of N that cannot be precipitated and fixed is 30 ppm or less.In ordinary aluminum killed steel, 30 ppm of N has no adverse effect, but when Ti is added in combination, AtN precipitates using TIN as precipitation nuclei (Ti, At ) Since a composite precipitate of N is formed and becomes a stable precipitate even at extremely high temperatures, it was found that it has the same effect as precipitating substantially all of the N as TiN. For smoothing, it is desirable to add 0.002% or more of Ti.Additionally, if it is added in an amount exceeding the equivalent of the sum of C and N, the properties similar to those of Ti-added steel will be strong, and secondary processing cracks will easily occur. Therefore, the upper limit is set to less than πC (% deviation l 4 N (support)).From the viewpoint of ductility, yield strength, and economics, the amount of Ti added is 0 at 14 N (ti) or less, which does not generate Tic.
．． 025 or less is most preferable.

The amount of Nb added is 2C← in order to form a composite precipitate.
), and if it is less than 0.0031, the effect is small, and if the amount of Nb added is 0.04% or more, there will be no precipitates with a composition close to that of NbC, which is a disadvantage of steel with only Nb added. It doesn't seem like it's going to show up clearly. Most preferably, it is bundled with an added amount of Nb<0.025%.

Cold-rolled steel sheets are treated with phosphate (?) as a base treatment for painting.
However, it must also have excellent bonding properties. However, in ultra-low carbon steel, N
Addition of b+Ti has the property of significantly degrading bonding properties. In particular, it is necessary to clean the welded part with a grinder to ensure good chemical conversion properties even in areas where new surfaces are exposed.
d Tl, Nb addition amount as Ti (%) + Nb (@< 0
．． It is necessary to limit it to 0.6%. The most desirable range is Tj (%) + Nb (excellent) (0.05 chi).

Next, the range of elements other than B, Ti, and Nb will be described. When the amount of C is large, Nb inevitably fixes C.
The amount is large, the manufacturing cost is high, and the amount of composite precipitates produced increases, leading to a decrease in multi-material properties where precipitation strengthening factors are too strong. Therefore, it is set to o, o s% or less.

Si may be added when making a high-strength steel sheet, but it is an element that promotes brittleness and inhibits chemical conversion treatment, so the content should be 0.8 or less. Mn can also be used to increase the strength. However, because it has the property of deteriorating the r value and the cost of ferroalloy is high, 1. Keep it below Oq6. P is the element with the greatest strengthening ability and is added to increase strength, but if it is included in a large amount, the amount of grain boundary segregation increases and causes embrittlement, that is, secondary work cracking, so the upper limit is O. , 1%.

Substantially the total amount of N is fixed as (TI 2 , At)N, but if the N content is large, a large amount of Tl is required to be added, so it is set to 0.005% or less. 50 pp of C and N
By limiting the amount to an extremely low amount of less than m, the amount of precipitates is reduced, the ductility is good, the yield strength is low, and T f , N
When the amount of b added increases, the negative effects tend to be alleviated.

Next, the manufacturing conditions will be described.

In the steel according to the present invention, N is rendered harmless by precipitating and fixing it with Ti, and due to the gradual addition of Ti and Nb, (Ti, Nb)C composite precipitates are precipitated at high temperatures. However, by setting the hot rolling heating temperature to 1300° C. or lower, these precipitates or precipitation nuclei are sufficiently present in the heating furnace. As a result, it has become possible to precipitate substantially the entire amount of N as (Ti, AL)N with the small amount of Ti added as shown in the claims, and (
TI, Nb)C composite precipitates begin to precipitate in the high temperature range before finishing. Therefore, we have obtained the new knowledge that even at a low coiling temperature, the precipitates are slightly aggregated in the hot rolled sheet state, and the material behavior is insensitive to the coiling temperature. By limiting the heating temperature to 1,300° C. or less, the agglomeration of precipitates is improved and the adverse effects thereof are reduced, so the upper limits of the amounts of Ti and Nb added are also slightly relaxed. In addition, since the material characteristic VCr value is improved, it has a positive effect on secondary workability, and the effect of B addition is remarkable and is sufficiently effective when added in an amount of 2 ppm or more.

Promoting the coarse aggregation of precipitates also has a positive effect on chemical conversion treatment properties, relaxing the upper limit of the total amount of Ti+Nb added. In other words, precipitates such as (Ti, Nb) C and (Ti, At) N are less soluble in acids than Fe s C, so phosphate crystals are difficult to precipitate, which adversely affects chemical conversion treatment properties. However, by agglomerating it, it can be
□The precipitate density decreases, and chemical conversion treatment properties improve.

In the present invention, there is no need to particularly specify other hot rolling conditions. However, since the r value and Et tend to decrease as the hot rolling finishing temperature decreases, a finishing temperature of 850° C. or higher is preferred. Although there is no need to specify the winding temperature for the above reasons, the winding temperature is preferably 600° C. or higher and 800° C. or lower in order to improve the adhesion and secondary processability of the plating layer.

There is no need to particularly specify the cold rolling conditions. Deep drawing resistance tends to improve as the cold rolling rate increases, and secondary work brittle cracking is less likely to occur as the r value of the steel sheet increases. 5 for
A cold rolling ratio of 0% or more is most preferred. Steel Vi according to the invention
Although the recrystallization temperature is low because only a small amount of Ti and Nb is added, increasing the cold rolling rate is effective in further lowering the recrystallization temperature and lowering the annealing temperature.

Regarding the annealing conditions, continuous annealing is performed at a temperature above the recrystallization temperature and below Acy point. Since the cooling rate of box type annealing is extremely slow, diffusion of P into the grain boundaries occurs during cooling, which is undesirable9. When producing hot-dip galvanized steel sheets according to the present invention, as mentioned above, the temperature is 600°C or higher after hot rolling. Winding is performed at a winding temperature of 800°C or less, and the temperature range from 700°C to 500°C is 2°C/sec or more and 150°C/B during the cooling process after annealing.
It is preferable to cool at a cooling rate of ee or less. T.I., B.
is concentrated on the surface during the annealing treatment and subsequent cooling process, and forms a (Tt, B, zn) alloy phase when hot-dip galvanizing is performed. This alloy layer is brittle and causes a phenomenon in which the plating layer brittlely peels off during press molding. By winding at a winding temperature of 600°C or more and 800°C or less, the hot rolled sheet has T1. Since B is concentrated on the surface and the concentrated layer is removed in the descaling step, surface concentration of T i+ H in the annealing step is suppressed and good adhesion is obtained. By adopting such a winding temperature, 9B
It is also advantageous for secondary workability because it already concentrates at the grain boundaries in the hot-rolled sheet. TL at winding temperature below 600℃
If the surface concentration of B does not occur sufficiently and the reverse trap temperature exceeds 800°C, the oxide film will be extremely thick, and parts that cannot be removed locally during the descaling process will easily remain, resulting in poor plating adhesion. If the cooling rate from 700°C to 500°C in the cooling process after annealing exceeds 150°C/1H1e, the shape of the steel sheet during plating will deteriorate, the adhesion of the plating layer will be poor, and B will become concentrated at grain boundaries. Since this is difficult to occur, the effect of improving primary workability is small. On the other hand, if it is less than 2° C./see, secondary processing cracks are likely to occur due to grain boundary segregation of P, and it is not economically efficient and inappropriate.

The technical concept of the present invention is applicable to the production of surface-treated steel sheets such as cold-rolled steel sheets, hot-dip aluminized copper sheets, tin-plated steel sheets, and chrome-plated steel sheets manufactured by continuous annealing, as well as ultra-thin steel sheets due to the low recrystallization temperature. Applicable.

Examples of the present invention will be described below.

Example 1 Steel slabs having the components shown in Table 1 were melted and hot rolled under the hot rolling conditions shown in Table 1. The finishing temperature is 89 in both cases.
The temperature is 0 to 910°C. The hot rolled plate thickness is 3.8mm.
After pickling and cold rolling to a thickness of 0.8 a, a single-sided hot-dip galvanized steel sheet was produced on a continuous hot-dip galvanizing line. Annealing cycle is 780-820℃ at approximately 10℃/8e
After heating to 40 seconds and maintaining the temperature range for 40 seconds, it was cooled to room temperature at an average cooling rate of 50 to 1.00° C./[IeC. FIG. 1 shows an annealing cycle.

After applying 0.8% skin pass i, the material was subjected to a material test and the results are shown in Table 2 together with the results of the chemical conversion treatment and secondary processing cracking tests. For some materials with high winding resistance, the materials at positions corresponding to the longitudinal center (upper row) and longitudinal ends (lower row) of the hot-rolled coil are shown.

**), ****) Chemical conversion treatment method and evaluation (1) As for the sample material **) For K, the Fe side of the Skinpass bottle was used. ***) VC was performed when the grinder was cleaned and the new surface was exposed.

(2) The treatment liquid is phosphofluorite (Zn2F8(P)
O4) 2) GrSD made in India in Japan with soaking treatment type softener
-2000 was used. This is AT16-18. zn+
1ooo±200ppm + Fe 50~loOp
The test was carried out by immersing the sample in a pmK-adjusted solution for 120 seconds.

(3) Evaluation was carried out by determining the density and size of phosphate crystals using a scanning electron microscopy photograph at a magnification of 91,000 times. (○: good, △: defective in some parts, ×: poor) Copper according to the present invention (sample steel Al-5) all showed good results. Test steel A6 showed Ti, Nb Chemical conversion treatment properties are poor due to the large amount of added steel (Tj ((6)>0.06%).
The composite addition effect of j is small, and the material quality and secondary workability are inferior compared to AI. Since A8 does not contain B, secondary processing cracks are likely to occur, and on the other hand, A9 has too much B added, resulting in Y
P, Et, r values are not good. A 10 i'i Since the amount of Ti added is large, the properties are similar to those of KSTl-added steel, and the secondary workability and chemical conversion treatment properties are inferior. Since Al has a small amount of Nb, it has a large amount of solid solution C, and has a high aging property and is inferior in material quality. A
No. 12 contains too much Nb and has a material similar to Nb-added steel, and a good material cannot be obtained at a winding temperature of less than 00°C. A
I 3-15FiTi is not added, in this case H
Since B forms BN with N, B has no effect of improving secondary workability (A13).

Further, when the winding temperature is low (A 14 ), the K material deteriorates significantly. If the amount of B added is increased as in Al 5, the secondary workability is improved, but the material quality is inferior. A l 6, l 7
is the case where only B was added without adding Nb to the T1-added steel; in this case, B was added. Material deterioration is large, the effect of improving secondary workability itself is small, and chemical conversion treatment properties are also poor.

Example 2 A steel slab having components A2 and 4 shown in Table 1 was hot rolled at a heating temperature of 1200°C, a finishing temperature of 900°C, and a coiling temperature shown in Table 5.3. It was a coil with two pressures. After pickling and cold rolling to obtain a coil of 0.8 mm, an alloyed hot-dip galvanized steel sheet was manufactured using the cycle shown in FIG.

The average cooling rate from 700°C to 500°C is shown in Table 5. Table 5 shows the plating layer adhesion and secondary workability of the hot-dip galvanized steel sheets obtained under these conditions. The tensile property values were omitted because they showed almost the same values as the results in Tables 2 and 4.

Table 3 Note) *) Test method and evaluation are the same as in Example 2.

**) Metal material bending test (JIS Z2248) IT bending, the bent circular surface was peeled off with a tape, and the adhesion was evaluated by the amount of the plating layer attached to the tape.

When the coiling temperature is less than 600°C, Ti
, B does not concentrate on the surface, resulting in poor adhesion. 8
When the coiling temperature exceeds 00°C, adhesion is poor because there are localized scale residues during pickling of ll-. In addition, the cooling rate from 700 to 500°C is 2°C/se.
If it is less than C, grain boundary concentration of P tends to occur during cooling, resulting in poor secondary workability, and surface concentration of pTI and H tends to deteriorate adhesion. On the other hand, when the temperature is 150° C./Bec or higher, B is difficult to concentrate at grain boundaries, so the effect of improving secondary workability is small, and the plate shape is not good, resulting in poor adhesion.

Therefore, in order to obtain a hot-dip galvanized steel sheet with good coating layer adhesion and excellent secondary workability, the coiling temperature should be set at 600°C.
Above 800℃, the cooling rate after annealing is 2℃/
It is necessary to keep the temperature above 150℃/B e C.

Example 3 A steel slab with the composition shown in Table 4 was heated at a heating temperature of 1180℃.
It was hot rolled at a finishing temperature of 890°C and a winding temperature of 680°C to form a coil of 3.81 mm. After pickling and cold rolling to make a 0.8fl coil, it was subjected to recrystallization annealing, two-alloying hot-dip galvanizing, and a skin pass of 0.8 in the cycle shown in Figure 2, and then subjected to material testing. did. The results are shown in Table 5.

Conventionally, high-strength steel sheets with high r-values are TS-40 with 9f/
The limit was m1l12 class. This requires the addition of reinforcing elements such as P and Si to K in order to give it further strength, but this is hindered by the fact that these elements significantly promote embrittlement and are likely to cause secondary processing cracks. Ta. It also had the disadvantage that if an attempt was made to improve the secondary workability by adding B, the material would deteriorate significantly.

As shown in Table 5, when B is added to the conventional steel with only Ti and Nb added, the material quality deteriorates significantly, and the effect of improving secondary workability is very small with a small amount of B. The steel of the present invention has extremely excellent secondary workability even with a small amount of B added, and from the viewpoint of material quality, B addition and P.

There is no adverse effect of adding Si or Mn. Therefore, the steel of the present invention is extremely advantageous in the production of high-strength steel plates and thick steel plates that are susceptible to secondary workability. In addition, the plating adhesion is extremely good.

[Brief explanation of the drawing]

FIGS. 1 and 2 are explanatory diagrams showing a heat treatment cycle in an embodiment of the present invention. Figure 1 Figure 2

Claims (1)

[Claims] (i) c: 0.005% or less, St: o, s
% or less, Mn: 1.0% or less, P: o, i% or less,
At: o, 0.1 to 0.1%, N: 0.005% or less and other unavoidable impurities, and B, Tt
, the essential condition is to add Nb in combination, and B is 3
Added within a range of 0 ppm or less, contained within a range that satisfies the condition of +48N (A), Nb is 4 Nb (→) 2 C (%), and 0.003 Ch≦Nb
(a) The content is within the range of dropping <0.04%, and T
Steel with components satisfying i(%1l)-1-Nb(a)<0.06% is hot-rolled at a hot-rolling heating temperature of 1300°C or less, then subjected to descaling treatment, and continuous melting after cold rolling. Molten zinc for ultra-deep drawing, which has extremely excellent secondary workability and is characterized by continuous annealing at a temperature above the recrystallization temperature and below A'c 3 on a galvanizing line, and then hot-dip galvanizing. Method of manufacturing plated steel sheets. (2) After hot rolling, the coiling temperature is 600°C or higher and 800°C or lower, followed by cold rolling, and annealing at a temperature higher than the recrystallization temperature and lower than the AC6 transformation point in a continuous hot-dip galvanizing line. 700℃ to 500℃ after treatment
2. The method according to claim 1, wherein the temperature range is 2° C./86° C. or more and 150° C./BeC or less at a cooling rate, and then hot-dip galvanizing is performed.